Conventional valves include those used in fluid catalytic cracking systems in petroleum refineries for controlling the flows of gas and powdery catalyst, those installed in pipe channels for conducting dust-containing flue gas from blast furnaces to boilers utilizing the gas pressure, and those used for controlling the flow of gas containing a large amount of particles or dust or the flow of high-velocity fluid.
These valves include rotary valves. With reference to FIG. 5, a rotary valve 1 disposed between pipes 11 and 12 comprises a valve case 2 and a cylindrical valve element 13 provided within the valve case 2. The valve element 13 is turnable through a suitable angle to alter the degree of opening of the fluid passage 16 and to thereby control the flow of the fluid passing through the passage 16. The valve element 13 in the illustrated position has a fluid inlet 14 and a fluid outlet 15 which are out of alignment with the fluid passage 16, causing the fluid to flow as deflected along the arrows shown. The flow of the fluid therefore involves a great pressure loss. Especially since the fluid strikes the inner wall of the valve case 2 and then flows out from the outlet 15, the fluid striking portion a of the inner wall wears away rapidly, consequently reducing the life of the valve. At the inlet side of the valve element 13, the fluid impinging on the peripheral surface of the valve element 13 wears away a portion b of the valve element 13, so that there is the necessity of reinforcing this portion. However, since the peripheral surface of the valve element 13 is adapted for sliding contact with the inner surface of the valve case 2 when the valve element 13 is turned, it is impossible to provide an abrasion resistant lining on the surface of the valve element 13. Thus, the valve element 13 also involves the problem of early wear.
FIG. 7 shows a fluid catalytic cracking (FCC) system in which crude oil, kerosene, gas oil or like heavy oil in the form of vapors is brought into contact with powdery catalyst such as silica alumina at a high temperature within a riser 77 and cracked in a reactor 71. The cracked vapors are further processed to obtain propane gas, olefin gas, high-octane gasoline and other refined products. The particles of the catalyst coated with carbon during use are drawn off from the reactor 71 and returned to a regenerator 72, in which the carbon is burned off. The regenerated catalyst is circulated through the riser 77 again.
The flue gas channel 76 of the regenerator 72 is provided with a pressure difference regulating valve 75 for controlling the discharge of the flue gas as well as the pressure difference between the reactor 71 and the regenerator 72. The line 73 for returning the used catalyst from the reactor 71 to the regenerator 72 and the line 74 for guiding the regenerated catalyst from the regenerator 72 to the riser 77 are provided with flow control valves 1 and 1. Valves heretofore used in the FCC system comprise slidable valve plates 7 and 7a having opposed end edges which are brought into abutting contact with each other at the center position to close the flow passage (see FIG. 6). Since the valve plates are of the slider type, it is impossible to provide an abrasion resistant lining on the sliding portions, so that the valve plates wear away rapidly, reducing the life of the valve. Furthermore, the valve plates, which are adapted to block the flow of the fluid at right angles thereto, produce a marked turbulence leading to increased wear and entail a great pressure loss. The valve is therefore disadvantageous for use in the FCC system in which it is frequently used in its half-open position.
Additionally, since seat rings and guide members which are fastened by bolts or the like are exposed to the fluid, the theaded portions are oxidized, scorched or otherwise damaged owing to the influence of the high temperature, consequently presenting extreme difficulty in removing the parts.